It can also be due to the wiring from the circuit board to the sensor making contact with each other, there is a you tube vid on this. Apparently there are three wires, the middle one is properly insulated, but the two side wires only have a thin coating which can wear off over time, and make momentary contact with sections of the board. The fellow who fixed his propointer desoldered the two side wires, and slipped on a couple sections of shrinkwrap on to prevent unwanted contact. Something to try if your propointer is out of warranty.
You can also buy new switch kits off ebay if you feel confident of having a play with it.
There was also an issue with components coming adrift in the actual sensor, but if this is the case, then it is probably a throw away item if not unded warranty. A lot of the earlier propointers had this issue, and Garrett simply swapped the pointer out for a new one as it is unrepairable. When I visited Phil at the Gold Mining Centre, he had a whole bucket full of defective propointers that had this problem.
The battery not seated properly was another common issue, same goes for the Pro-find if the cap is not screwed on tight enough, or more to the point, the foam pad inside the lid was too crushed over time to give a the battery a good contact. The Pro-find switches also have a tendancy to "catch" if the rubber gets pinched in the recess, making continuous contact on the electronics.
I see the new Whites Pinpointer may have resolved some of these issues by making the whole case interchangable vs having the sensor permanently glued into the casing.